My first DT project
The rules are:
Must be smaller than 400x150x150.
Choice of materials : Steel, ally, wood, perspex.
Adam
What material to make the chassis out of?
My first DT project
The rules are:
Must be smaller than 400x150x150.
Choice of materials : Steel, ally, wood, perspex.
Adam
i would make the part that holds the rubber band out of steel, anything else out of ally where possible.
sounds fun!
the aim must be to store as much energy as possible and use it to propel as light a car as you can manage!
[Edited on 7/12/04 by JoelP]
It will be. Thanks Joel.
Was thinking about the weight of ally compared to perspex/wood.
Adam
I would go for balsa wood light easy to cut etc.
Its what we used to make rubber powered planes from.
Cheers,
Bob
good point about balsa bob. i compared steel to wood etc but forgot to consider the ally alternatives!
variable wheel size, depending on the length track.
hey, wouldnt it be fun to have a LCB laggy band dragster race! kids get all the fun...
joel>>> off to toys'r'us now
I did this as a first year uni project, we made ours out of balsa wood with big foamboard wheels - very light and low moment of inertia on the wheels.
We also put rubber bands around the wheels to make a basic tyre to give a bit more grip.
One thing to look out for is the way you design the power mechanism - if you're not careful the rubber band will put all its energy into getting
the car moving, and then wrap itself up the other way and slow the car down. Quite a few teams looked to be doing pretty well only to have their car
suddenly stop and reverse back along the track!
Pete
Drive it with a propellor and then it will freewheel after the laggy band runs down.
If you don't actually attach the laggy band to the rear axle, but just wrap in around once so it effectively holds itself on, then when you run out of "tug" the band will just drop off and the car should carry on ?
Thats what I was going to do. A propellor would just be more weight.
Adam
I did it too when I was at school. the rear axel was a length of dowel with a blunted panel pin hammered through it. The elastic band was hooked onto the pin which allowed it come loose when it had turned the axel the appropriate number of times.
Yeah we did ours by hooking the band onto the axle so it would come loose, but some people tied theirs solid to the axle. idiots... 
Pete
Some good ideas coming. Any other tips?
Adam
Would a flywheel be any good?
do it downhill?
an ABS or perspex chassis may be a little more sturdy than balsa , depends how long its got to last , wether the opposition has done it etc .
For wheel bearings , try to get the least friction , for a radio controlled car project , the axles i used were mounted in thin ally to reduce
friction as much as possible , fairy liquid may help , altho it can be too viscous , maybe water it down a bit , or use some small amoumts of
molybendum
quote:
Originally posted by phelpsa
Some good ideas coming. Any other tips?
Adam
6 meters, quickest person wins.
Adam
Does the power unit have to be in the car? I'm thinking big catapult
Yep. Must be in the car.
Adam
Well, just make sure you engineer it to extract all the energy from the rubber band. Is there a standard "control" rubber band that
everyone will be issued with, or is it a free for all? If so, obviously get the longest, thickest one you can.
You need to design the chassis so you can put the maximum amount of stretch into the whole band, not just part of it (i.e. don't wrap the rubber
band around the back axle, a big chunk of the band won't be doing anything usefull). If the band at max stretch won't fit in the length of
the vehicle, consider a low friction pulley arrangement to fold the band one or more times up and down the car. Use talcum powder to lubricate the
band on pulleys etc.
Be carefull how you attach the ends of the band, don't want any sharp edges that could cause failure.
Be very carefull working out the gearing. If you have it wrong the car will either sit spinning it wheels until most of the bands energy has gone, or
will trundle off the line like a tortoise.
i was gonna mention a CVT type drive, with a cone shaped spinner. and maybe a bike pedal type device to allow it to free wheel after the band is
unwound.
does it follow a rail or is self centering an issue?
[Edited on 7/12/04 by JoelP]
I was thinking something like this. The peg would be enough to hold the elastic band on but not keep it on, so it would come of when it runs out.
Adam
Rescued attachment untitled.JPG
P.S. the outer bit of the cone would act as a flywheel.
Adam
its gonna take many experiment runs, to hone it to the track length. i wouldnt put extra weights in to make a flywheel - if the rubber band is spent
by the end, you havent wound it up enough! hence the flywheel is extra mass (both static and rotational).
bear in mind, in the diagram above, when the band has wound itself down to the narrow part, you need max torque to to keep it going. But the band will
barely have any pull left.
the benefit of twisting it end to end, is that it has a uniform 'powerband'. but then, the axis of rotation is wrong. and, its harder to do
the CVT approach.
also, if the band lasts til the end of the run, it doesnt matter if it gets wound back up in reverse.
I've got a couple of months to complete it so I expect there will be a lot of tests. I will have to try both approaches and see which one is
best. Just as I finished that post my DT teacher (who is on duty in the house) came in and said about torque being the problem and that I would have
to be very clever to get it right.
Adam
P.S. The outer would ACT loke a fly wheel but I wouldn't add anything to it.
good thinking.
Did same thing at uni in freshers week as a get to know people thing. Some of the technicians were recommending using condoms stretched over the wheels for tyres....... Never tried it as it was suggested too late so can't comment on how effective they would be.
It might be a bit embarassing asking our DT teacher for condoms.
Adam
Here's a challenge guys how about the Locost builders laggy band drag race.We could have it on the first big kit car show of next year.
Perhaps one of the Locost suppliers could donate a prize and we have a nominal entry fee that went to charity.
Cheers,
Bob
[Edited on 8/12/04 by splitrivet]
Good idea!
wouldnt be fair, adam gets a test run...
In a clock it's called a fusee (there's a french accent in there somewhere! ). You could make one of them out of balsa, and it would
probably work very well.
High torque at the start and high gearing at the end.
David
quote:
Originally posted by phelpsa
I was thinking something like this. The peg would be enough to hold the elastic band on but not keep it on, so it would come of when it runs out.
Adam
Yep, thats what my DT teacher called it, and he told me all about the springs in watches working like that aswell.
Adam
What material for the wheels? Bulsar wood?
Adam
Balsa rims and cotton spokes
ATB
Simon
What about this design? It would be easy to make (lathe and drill).
Adam
Rescued attachment Wheel.JPG